Affliction and Destruction have a 15 minute CD on the guardians. This reduction simply means they get to use their guardians almost every pull.
Demonology, on the other hand, has a 10 minute CD. During Longer fights, it will be able to use the guardian TWICE. It already gets at least triple the benefit from it's guardian demons. Now it will get 6x the benefit during long fights.
THEN the bonus increases the duration by 15 seconds for Destro and Aff, and TRIPLE that for Demo.
Now were talking about the 2pc bonus favoring Demo over either of the other specs by a factor of at least 18 during fights lasting longer than 7 minutes.
I'm sorry, but WTF?
Lets work this out another way:
Assuming a 7:30+ fight, The 2pc bonus increases the uptime of the doomguard by 15 seconds for aff and destro, and 155 seconds for demo. That's already 10x the benefit for demo.
The doomguard does about 3-4x the DPS for Demo as well.
.... lol. Should really revise before you post